1. Field of the Invention
The present invention relates to gyroscopes, and in particular to improved resonator microgyroscopes and their manufacture.
2. Description of the Related Art
Gyroscopes are used to determine direction based upon the sensed inertial reaction of a moving mass. In various forms they are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to determine the orientation of a free object.
Older conventional gyroscopes were very heavy mechanisms, employing relatively large spinning masses by current standards. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as vibratory gyroscopes.
Spacecraft often depend on inertial rate sensing gyroscopic systems to supplement attitude control. Typical systems employ conventional spinning mass gyroscopes or conventionally-machined hemispherical resonator gyroscopes to provide the high pointing stability required for spacecraft payloads operating high above the earth. However, both of these types of gyroscopes are expensive, large and heavy.
Some prior symmetric vibratory gyroscopes have been produced, however, their vibratory momentum is transfered directly to their baseplates or packages. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation. In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and vibrating ring gyroscope, which can have desirable isolation and vibrational symmetry attributes, are not suitable for thin planar silicon implementation with sensitive electrostatic sensors and actuators that take advantage of the large planar areas of the device
The scale of previous silicon microgyroscopes (e.g., U.S. Pat. No. 5,894,090) has not been optimized for navigation grade performance resulting in higher noise and drift than desired. This problem stems from use of thin eptiaxially grown silicon flexures to define critical vibration frequencies that are limited to 0.1% thickness accuracy and limit device sizes to a few millimeters. The former results in high drift due to vibrational asymmetry or unbalance and the latter results in high rate noise due to lower mass increasing thermal mechanical noise and lower area increasing capacitance sensor electronics noise. Scaling up of non-isolated silicon microgyros is also problematic because external energy losses will increase with no improvement in resonator Q and no reduction in case-sensitive drift. An isolated, cm scale resonator with many orders of magnitude in 3D manufacturing precision is required for navigation grade performance. Conventionally machined navigation grade resonators such as in hemispherical or shell gyros have the optimum scale, e.g. 30 mm and 3D manufacturing precision and hence desirable drift and noise performance, however are expensive and slow to make. Conventional laser trimming of mechanical resonators can further improve manufacturing precision to some degree, however it is not suitable for microgyros with narrow mechanical gaps and has limited resolution necessitating larger electrostatic bias adjustments in the final tuning process.
There is a need in the art for small microgyros with greatly improved performance for navigation and spacecraft payload pointing. There is also a need for such gyros to be cheaper and more easily manufactured with greater 3D mechanical precision. Finally, there is a need for such gyros to have desirable isolation and vibrational symmetry attributes while being compatible with planar silicon manufacturing. The present invention satisfies all these needs.
The present invention discloses a resonator gyroscope comprising a resonator including two bodies, each with a center of mass and transverse inertia symmetry about an axis that are substantially coincident and each supported by one or more elastic elements and wherein the bodies together form two differential rocking modes of vibration transverse to the axis with substantially equal frequencies and transfering substantially zero net momentum to the baseplate. The gyroscope further includes a baseplate affixed to the resonator by the one or more elastic elements and sense and drive elements each affixed to the resonator and baseplate. An isolated resonator microgyroscope is thereby provided having no coupling of its sense or drive mode to baseplate or package motion except through Coriolis accelerations when a differential rocking mode is internally driven.
One embodiment of the invention comprises a resonator including a proof mass and a counterbalancing frame affixed to the proof mass by one or more elastic elements. The resonator is affixed to a baseplate by the one or more elastic elements. Sense and drive elements are each affixed to the resonator and baseplate. The proof mass and counterbalancing frame produce substantially no net momentum transfer or reaction on the baseplate when a resonator differential rocking mode is excited. The resonator excluding central proof mass may be etched from a single thicksilicon wafer or from a thin silicon wafer to which is bonded a post proof mass.
An all-silicon, symmetric vibratory gyroscope of the present invention is inexpensive to produce using photolithography and because of its unique isolated design can be scaled large enough (e.g. 20 mm mesoscale resonator) to achieve the low noise and low drift performance required for navigation. Combined with a low cost closed loop analog control system, a mesoscale navigation grade inertial reference unit would be very inexpensive for relatively small manufacturing quantities. Furthermore, combined with a low-power digital control electronics application specific integrated circuit (ASIC) for much larger volumes, a xe2x80x9cgolf ball-sizedxe2x80x9d inertial navigation unit is feasible. For example, in unit quantities of approximately 3,000 per year, it is estimated that such a unit would cost less than $3,000.
The present invention provides an affordable vibratory gyroscope with navigation grade performance by means of a precision isolated symmetric planar resonator of optimum scale that can be fabricated with silicon photolithography from commercial double-side polished silicon wafers with low total thickness variation. Previous navigation grade vibratory gyroscopes with isolated resonators have relied on conventional lathes or milling machines yielding slow and expensive 3D precision machining and assembly, e.g. quartz hemispheric resonator gyroscopes, or employed non-isolated resonators mounted on low-frequency isolaters to gain a degree of isolation at the expense of increasing seismic suspension mass and increased deflections due to gravity loads. Asymmetric tuning fork vibratory gyroscopes provide isolation about the drive axis only and are subject to external disturbance about the output sense axis. The cloverleaf microgyroscope of U.S. Pat. No. 5,894,091 as previously mentioned is subject to external disturbances about its drive and output axes.